The present invention relates in general to photolitho-graphic processes. Specifically, the present invention is concerned with a process for the manufacture of metallic structures in a substrate having narrow dimensions.
In semiconductor technology and in microelectronics, the dimensions of structures are becoming smaller and smaller. In memory production today, e.g., structures with a width of less than 200 nm are produced using optical lithography in combination with the masking technique. Photholithographic processes are vital steps in the fabrication of, e.g., semiconductor devices. In a photolitho-graphic process, an exposure light, usually ultraviolet (UV) light is used to expose a photoresist-coated semiconductor wafer through a mask (in the following called photomask). The purpose of the photolithographic process is to transfer a set of patterns representative of the circuit layer onto the wafer. The patterns on the photomask define the positions, shapes and sizes of various circuit elements such as diffusion areas, metal contacts and metallization layers, on the wafer.
In optical lithography a limit can be expected at approximately 70 nm because of diffraction effects.
However, structures with even smaller dimensions are required for new applications such as single-electron transistors or molecular electronic components. In the case of very high-frequency circuits this is also true in conventional electronics. There is also a need to reduce, e.g., the read and write dimensions in thin film magnetic heads. In addition to that, micro structures having a very high aspect ratio of about 5 to 30 and greater will be needed.
Today's photolithographical techniques are still restricted by the wavelength of the used exposure light to arrive at critical dimensions as small as possible. Reduction of the critical dimensions was done in most cases by the reduction of the wavelength of radiation, i.e., starting with UV exposure and proceeding to DUV exposure, electron radiation and X-rays. X-ray lithography, e.g., makes it possible to image dimensions of less than 100 nm. In electron and ion beam lithography, structures as small as 10 nm can be generated with high-energy particles. However, this requires expensive vacuum systems and beam guidance systems. In addition, problems can occur with sensitive components due to radiation damage in the substrate, because the high-energy particles can penetrate through the resist layers required for etching processes.
U.S. Pat. No. 5,837,426 discloses a photolithographic process which provides reduced line widths or reduced interelement line spaces for the circuit elements on an IC chip, allowing the IC chip to have a higher degree of integration. This photholithographic process includes a double-exposure process on the same wafer defined by placing either the same photomask at two different positions or by using two photomasks.
In U.S. Pat. No. 6,042,993 a photholithographic structure generation process for structures in the sub-200 nm range is disclosed wherein a layer of amorphous hydrogen-containing carbon with an optical energy gap of <1 eV or a layer of sputtered amorphous carbon is applied as the bottom resist to a substrate; the bottom layer resist is provided with a layer of an electron beam-sensitive silicon-containing or silylatable photoresist as the top resist; the top rsist is then structured by means of scanning tunneling microscopy (STM) or scanning force microscopy (SFM) with electrons of an energy of ≦80 eV; and the structure is subsequently transferred to the bottom resist by etching with an anisotropic oxygen plasma and is next transferred to the substrate by plasma etching.
On the other hand, printing from a patterned surface to thin layers of material is a well known and well documented process in printing industry.
Printing processes were originally developed for the exchange and storage of information adapted to human vision. This field of application requires pattern and overlay accuracies down to 20 μm for high-quality reproduction. In a few cases, printing processes have been used for technological patterning, e.g., gravure offset printing was used to make 50-μm-wide conductor lines on ceramic substrates, and to pattern thin-film transistors for low-cost displays. Offset printing was used for the fabrication of capacitors and printed and plated metal lines as narrow as 25 μm. Finally, printed circuit boards and integrated circuit packaging are popular applications of screen printing in the electronics industry. (B. Michel et al., IBM J. Res. Develop. 45, 697 (2001) and references therein).
In a process known as flexography, viscous ink is printed onto porous paper and permeable plastic. Flexography is a direct rotary printing method that uses resilient relief image plates of rubber or other resilient materials including photopolymers to print an image on diverse types of materials that are typically difficult to image with traditional offset or gravure processes, such as cardboard, plastic films and virtually any type of substrate whether absorbent or non-absorbent. As such it has found great applications and market potential in the packaging industry. Usually, the viscous ink prevents a direct contact of the stamp with the substrate because it cannot be displaced quickly enough during the fast printing operations. The transfer of a thick layer of ink is desired in this typical mode of operation but also prevents replication of laterally small features—this is the main reason why printed feature sizes cannot be smaller than 20 μm. Printing onto metal foils has been implemented in a few applications but is much more difficult than other processes (H. Kipphan, “Handbuch der Printmedien”, Springer Berlin, 2000 and J. M. Adams, D. D. Faux, and J. J. Rieber, “Printing Technology 4th Ed.”, Delamare Publishers, Albany, N.Y.).
Microcontact printing uses a similar stamp as flexography does, but typically transfers a monolayer of ink onto an impermeable metal surface. A more general process now called soft lithography is successfully applied in different variants to print thiols and other chemicals to a wide variety of surfaces. Typically, the chemicals are first applied to the patterned stamp surface as solutions in a volatile solvent or using a contact inker pad. After inking and drying, the molecules are present in the bulk and on the surface of the stamp in a “dry” state and are transferred to the surface by a mechanical contact. Reasons for the choice of poly-(dimethyl)siloxane (PDMS) as the stamp material are its good rubber-like elasticity, a chemistry similar to glass, the possibility to buffer ink molecules, and—very important—its excellent gas permeability that enables small amounts of air to dissolve into or escape through the stamp matrix. (cf. B. Michel et al. “Printing meets lithography”, IBM, J. Res. Develop. 45 (5), 697 (2001)).
Since, as already pointed out above, processing of narrow dimensions, i.e., dimensions <0.15 μm, like future P2 width on wafers for standard magnetic recording heads using photolithography technology requires at least Extreme UV (EUV) or e-beam techniques followed by image transfer, there is a need for less expensive and, on the other hand, more robust techniques for fabricating narrow dimension structures on substrates. Additionally, there is a need for such structures that provide better distributions. Distribution of critical dimensions (CD) drives device performance and manufacturing yield and quality. In lithography, distribution depends on the tool, e.g., optics, flash fields, alugnment, etc., as well as the wafer surface flatness and process handling